1. Field of the Invention
The principle of electromagnetic position detection is well known for determining the position and orientation of a solid moving object in a reference frame. One of the applications of this principle is the determination of the direction of aim of a helmet aiming device which an infantryman or a driver or pilot of a tank or aircraft has placed on his head, in order to control with it a weapon, missile or control camera, for example.
2. Description of Related Art
This detection principle uses a magnetic field emitter or radiator, connected to the reference frame, or reference system, in which the measurements are being carried out, a magnetic field receiver or sensor, fixed or movable, whose position and orientation have to be determined, and electronic processing circuits including analogue amplifiers, a computing processor and processing algorithms.
The radiator must as far as possible satisfy the conditions of the dipole theory, in which the system of coordinates for the mathematical description of the radiation is a sphere centered on the dipole and the Green free space function depends only on the radial coordinate.
The magnetic field sensor has as far as possible to be confined to a point. The emitter radiates a field, sequentially or by multiplexing, along two or three orthogonal axes and the sensor detects sequentially the components of this field according to three or two orthogonal axes, the emission and reception each generally taking place along three axes. The sensor thus supplies, per emission axis, three measurements, i.e. nine in total, which are organized in a 3.times.3 matrix, from which the processing algorithms supply the position and orientation of the sensor with respect to the radiator.
It will be noted here that the determination of the position and orientation of the sensor entails the determination of six variables--the three cartesian coordinates, the relative bearing, the elevation angle and the roll--and that at least six measurements are therefore necessary. If transmission takes place only along two axes, reception must consequently take place along three, and vice-versa.
In a given reference system the magnetic field at a given point is represented by a vector H. In this reference system, the sensitivity axes of the sensor are represented by a vector C. The results of the measurements made by the sensor can be arranged in the form of a matrix M corresponding to the scalar product C.multidot.H EQU C.multidot.H=C.sup.T H=M
H being the matrix of the field and C.sup.T the transposed matrix of the matrix C of the sensitivity axes of the sensor.
The sensor, in the reference system, can undergo either a translational or rotational movement. Let us consider the latter, expressed by a matrix of rotation R. The matrix of the sensitivity axes of the sensor becomes RC and that of the measurements M.sub.R. EQU M.sub.R =C.sup.T R.sup.T H
If the three axes of the sensor are orthonormed, ##EQU2## Consequently EQU M.sub.R =R.sup.T H
H depending on the position of the sensor and M.sub.R on its position and orientation.
In the presence of magnetic perturbations, for example caused by the metallic mass of an aeroplane, the processing algorithms are based on cartographic surveys. In order to obtain this prior mapping, measurements of the field are carried out using the sensor at multiple points in space, with which are thus associated matrices of perturbed measurements. Let M.sub.C be one of these. If the sensor undergoes a rotation R, the matrix of measurements ought to become ##EQU3## The matrix product M.sub.R.sup.T M.sub.R is therefore rotation-invariant and representative of the point under consideration.
The mapping aims to determine the function f of correspondence between M.sub.C and M.sub.C.sup.T M.sub.C.
During subsequent measurements, the product M.sub.R.sup.T M.sub.R corresponds to a matrix M.sub.R.
As M.sub.R.sup.T M.sub.R =M.sub.C.sup.T M.sub.C, M.sub.C, and therefore the position of the sensor, is deduced from this through the function f. In order to determine the matrix of rotation R, that is to say the orientation of the axes of the sensor, knowing M.sub.R and M.sub.C, R is calculated from the equation M.sub.R =R.sup.T M.sub.C, i.e., EQU R=M.sub.C M.sub.R.sup.-1
Through the document FR-A-2 458 838 (791441), a system for the electromagnetic determination of the position and orientation of a moving object is already known.
Diagrammatically the radiator and sensor in this system are each formed by a group of three identical current-controlled coils arranged respectively along three orthogonal axes. In order to incorporate as well as possible the conditions of the dipole, for both the radiator and the sensor, the dimensions of the coil are as small as possible. It would also be possible to substitute for each coil two half-coils with the same current passing through them. It would also be possible to consider, as the radiator and sensor, a sphere made from magnetic material surrounded by three orthogonal coils.
With respect to the circuit for processing the signals delivered by the sensor, in the prior art system referred to above, and which aims to determine the amplitudes of these signals, it has one channel per axis, and on each channel, with reference to FIG. 1, an amplifier 1 and an analogue feedback loop 2, between a mixer 3 and a bus 4 on the computing processor 5, in order to avoid the phenomena of crosstalk coupling and to improve the accuracy and, in the mixer 3, to subtract from the signal delivered a signal Vref in phase with the latter, of the same frequency and almost the same amplitude. The feedback is controlled by software in the computing processor 5 continuously updated in order to take account of the previous measurement. The reference signal supplied to the mixer 3 is fixed by a digital-to-analogue converter DAC 6 controlled by the processor 5. The output signal from the feedback loop 2 is supplied to the computing processor 5 after conversion in an analogue-to-digital converter ADC 7. Between the mixer 3 and the converter 7, the loop 2 comprises an amplifier 8, a demodulator 9 and integrator 10. The demodulator 9 provides a synchronous and consistent demodulation and rectifies the signal coming from the mixer 3 by multiplication by the signal Vref of the same frequency and in phase. The integrator 10 integrates the signal from the demodulator 9 over a period determined by the processor 5 and the result of the integration is digitized in the AD converter 7.
But such a processing circuit has drawbacks. It has to be calibrated before each measurement. The input signals to the mixer 3 and the demodulator 9 have to be absolutely in phase.
The present invention aims to overcome these drawbacks.